Classic Paint for Windows 10 is the classic paint app before Paint 3D took its place. Great for those who miss it, or just for nostalgia.
Classic Paint for Windows 10 isn't portable, so installation is required. The nice part about the calculator is that it does not hijack the Windows 10 calculator, so you can always use whichever you want. It runs from the Start menu just like any other app when you need it.